One way to somewhat accomplish might be setting as a normal weapon attack, but leave the actual damage field empty and fill in the formula you want to use for damage dice in the "Content" field as an inline roll, something like '[[1d8+@{strength_mod}]] $type damage' (adjusting the damage die, mod variable, and damage type as you wish of course) which would at least prevent the damage dice from being doubled on a 20, and remembering to manually retrigger it on a 1